The same goes for the button that triggers walking. These are separate settings for horse and human walk and they cannot be set for the same (non-default) button inside the game menu.
However, there is a workaround for this problem. You can manually change the values for the buttons in the settings file.
..steamapps\common\Conan Exiles\ConanSandbox\Saved\Config\WindowsNoEditor\Input.ini
Before you 
start playing with it, it’s best to make a backup. 
